1. Field of the Invention
The present invention relates to a preferred signal display system applicable to image display apparatuses, display devices or television sets for displaying a plurality of video signals having different scanning frequencies such as video signals of the NTSC system, output video signals from personal computers (hereinafter, alternatively referred to as PC signals) and the like. More particularly, the present invention further relates to a multimedia type personal computer capable of outputting both PC display signals (such as VGA signals) and interlace scanning signals (such as an NTSC signal) converted to double-speed (hereinafter, alternatively called "double-speed signal"), and also relates to an image display apparatus possibly connected thereto for displaying both the image of a PC display signal and a double-speed signal, i.e., capable of displaying such differing signals simultaneously as well as at differing times. Especially, the present invention relates to an image display apparatus that has a high picture quality and which is capable of displaying both the image of a PC display signal and a double-speed signal, and to a personal computer connected to it.
2. Description of Related Art
A personal computer of a so-called multimedia type having a capability of outputting both television signals (such as NTSC signals) and PC display signals has been sold in recent years. However, an incompatibility problem arises in that display signals for NTSC system television sets use a 15.75 kHz horizontal deflection frequency, for example, while display signals for personal computers generally use a 31.5 kHz or more horizontal deflection frequency, for example. In order to solve such incompatibility problem, a multimedia computer may have a tuner that can receive broadcast television signals such as NTSC signals, and further may have a double-speed converter for converting a signal demodulated by the tuner to a double-speed signal.
Despite the ability to display the NTSC signal, when-an interlace scanning signals such as an NTSC signal is converted to a double-speed signal, it was found that a vertical resolution of this image declines. Still further, in a case where a still image is displayed, vertical resolution degradation becomes visible. The following background is helpful in understanding the resolution degradation problem associated with changing (e.g., doubling) a scanning speed of a display signal (e.g., when changing such scanning speed substantially to match the scanning speed of a second type of display signal and/or display image apparatus being used for display).
More particularly, attention is directed to FIG. 3 and to FIGS. 4a-4b representing an odd-field display frame 70 and even-field display frame 80, respectively, of an interlaced scanning arrangement. Such FIGS. 4a-4b will first be used to describe a normal interlacing operation and an exemplary resolution pattern (which has not experienced resolution degradation). More specifically, in FIG. 4a's display frame 70, assume for a moment that only solid scan lines 71, 73, 75 are present and represent original odd-field scan lines, and that such scan lines 71, 73, 75 are black lines. Similarly, in FIG. 4b's display frame 80, assume that only long/short dashed scan lines 81, 83, 85 are present and represent original even-field scan lines, and that such scan lines 81, 83, 85 are white lines. In normal interlacing (i.e., overlapping FIG. 4a's odd-field display frame 70 onto FIG. 4b's even-field display frame 80), the odd-field and even-field scan lines would be combined in an interlaced fashion to result in a display frame having the following arrangement of scan lines from top to bottom: line 71 (black), line 81 (white), line 73 (black), line 83 (white), line 75 (black), line 85 (white), i.e., to result in an exemplary (alternating) black-white-black-white . . . pattern (not illustrated).
Next, FIG. 3 and FIGS. 4a-4b will be used for purposes of discussion to describe a speed-changed, but resolution degraded, interlacing operation. More specifically, assume again that solid scan lines 71, 73, 75 are present within FIG. 4a and represent original black odd-field scan lines, and that long/short dashed scan lines 81, 83, 85 are present within FIG. 4b and represent original white even-field scan lines. For scanning speed doubling (e.g., when changing such scanning speed substantially to match the scanning speed of a different display signal and/or display apparatus), an original display scanning line waveform 60 (FIG. 3) and its corresponding original horizontal period waveform 61 are each duplicated (via double-speed conversion arrangements described ahead) to become double-speed signals 62 and 63, respectively, having horizontal scanning periods compressed to 1/2. More particularly, each single scanning line is duplicated into two scanning lines as illustrated in FIGS. 4a-4b.More specifically: original scanning line 71 now becomes dual scanning lines 71, 72; original scanning line 73 now becomes dual scanning lines 73, 74; . . . original scanning line 81 now becomes dual scanning lines 81, 82; original scanning line 83 now becomes dual scanning lines 83, 84; etc.
A problem, however, exists in that as a result of duplication, various odd-field scan lines and various even-field scan lines are now vertically positioned at positions within the odd-field display frame 70 and even-field display frame 80, respectively, such that odd-field scan lines and even-field scan lines will substantially overlap each other during an interlacing operation. For example, white odd-field scan line 72 in a preceding interlace frame will be overlapped by black even-field scan line 81 in a succeeding interlace frame, and vice versa. Such alternating overlapping of white scan lines and black scan lines between frames will appear to the human eye as a grey scanning line. Thus resolution degradation is experienced in that, instead of seeing the expected exemplary (alternating) black-white-black-white . . . pattern, a series of grey scan lines appear as a pattern.
Discussion finally turns to a speed-change operation which avoids the resolution degradation problem, i.e., via vertical shifting of selected scan lines. More particularly, like the immediately preceding example, each single scanning line is duplicated to form two scanning lines as again illustrated in FIGS. 4a-4b.However, this time to avoid the overlapping and resultant resolution degradation problem, a further vertical shifting operation is applied to each duplicate scan line, such that each original scan line and its duplicate scan line are caused to overlap one another. For example, in FIG. 4a, duplicate scan line 72 is vertically shifted (vector or arrow 77) substantially to overlap original scan line 71, using a one-half horizontal period waveform 64 (FIG. 3). Similar discussion can be made with respect to the other shift arrows 78-79 and 86-87. As a result thereof, when the FIG. 4a odd-field display frame 70 is overlapped with the FIG. 4b even-field display frame 80, the result is the FIG. 4c interlaced display frame 89 not having overlapped odd-field/even-field scan lines, and thus not experiencing a resolution degradation problem. More particularly, the interlaced display frame 89 has the following arrangement of scan lines from top to bottom: overlapped odd-field lines 71, 72 (both black), overlapped even-field lines 81, 82 (both white), overlapped odd-field lines 73, 74 (both black), overlapped even-field lines 83, 84 (both white), overlapped odd-field lines 75, 76 (both black), even-field line 85 (white), i.e., the original and expected exemplary (alternating) black-white-black-white . . . pattern is maintained.
One technique for overcoming the resolution degradation problem is disclosed in Japanese Patent Publication No. 6-14689. However, such technique suffers from at least two deficiencies. More particularly, first, in a case where a double-speed signal converted in advance is inputted from outside an apparatus (e.g., via an external input terminal), the arrangement disclosed in such publication cannot distinguish which raster scanning line must be vertically shifted in order to avoid the resolution degradation problem in an overlaying scan , i.e., all incoming scan lines appear the same to the apparatus as there is no information to distinguish shift scan lines from non-shift scan lines. Therefore, in a case where a double-speed signal converted in advance is inputted into the apparatus of such technique, necessary and proper vertical shifting cannot be determined or performed, and therefore vertical resolution of a corresponding image becomes degraded.
A second deficiency, while the above technique may work for a situation when one scanning speed is used consistently across an entire screen (FIG. 9), such background approach is disadvantageous when scanning speed and thus selective vertical shifting must be changed from area-to-area within a screen (i.e., intra-screen), for example, in a picture-in-picture operational mode such as illustrated in FIG. 9 which displays a PC display signal in an image b screen portion and an NTSC signal in an image a screen portion. More particularly, when an overlaying scan (including vertical shifting) is executed for an entire screen, for example, in a case where a signal of the image that is indicated in FIG. 8 is attempted, the vertical resolution of a PC image (image b) declines as selected scanning lines thereof are vertically shifted causing resolution degradation and/or scrambling of the display lines. To the contrary, when an overlaying scan (including vertical shifting) is not executed, the vertical resolution of a double-speed NTSC image (image a) declines as the resolution degradation problem discussed above occurs. Again, there is no information/arrangement to distinguish which portions of the picture-in-picture screen are speed-changed and/or need vertical scan-line shifting.